U.S. soldiers keep watch at the entrance of a military base near Alkozai village.

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An American soldier in Afghanistan allegedly walked off his base in the pre-dawn hours Sunday morning and began shooting at civilians in their homes in the southern province of Kandahar.

At least 16 civilians are reported dead, including nine children and three women. NATO hasn't confirmed the death toll, but has detained the accused service member.

The attack began around 3 a.m. in two villages in Panjwai, a suburb of Kandahar. They're not far from the U.S. base. As the AP reports:

The gunman went into three houses and opened fire, said a resident of Alkozai, Abdul Baqi, citing accounts from his neighbors.

"When it was happening in the middle of the night, we were inside our houses. I heard gunshots and then silence and then gunshots again," Baqi said.

Eleven of those killed were members of one family, many of them women and children.

After the shooting, NATO officials say, the soldier walked back to the base and surrendered.

Outrage over killing spree is rolling through Afghanistan. President Hamid Karzai called the shooting an "assassination" and called for an explanation from Washington. Villagers are calling for the U.S. to turn the soldier over to Afghanistan.

The incident makes a bad situation worse, renewing tensions that had just started to ebb over the recent burning of some Qurans.

President Obama and Secretary of Defense Leon Panetta have both reached out to Karzai to apologize for the incident and pledge their support for the Afghan people, but those actions aren't likely to calm things down.

As the situation continues to boil, we'll be updating the story here.

Update at 10:00 p.m. Soldiers at Joint Base Lewis-McChord react

Reporting for NPR, Northwest News Network's Austin Jenkins in Olympia, Washington says soldiers at the base where the alleged soldier was from are reacting with shock and dismay.

"It's going to be tough now," says Pfc. Jeremy Lozano, a medic getting ready for his first deployment for Afghanistan. "I mean it was already tough, but that's now more problems that we have to worry about."

Jenkins reports that soldiers also expressed concern about possible retaliation as a result of the killings.

Update at 6:13 p.m. Soldier said to be from Fort Lewis, Wash.

The Associated Press is quoting unnamed U.S. officials as saying the accused soldier is from Joint Base Lewis-McChord, Wash.

Additionally, NATO Secretary-General Anders Fogh Rasmussen expressed his "heartfelt condolences and sympathy to their families and loved ones, and to the Afghan people and government."

He added, "We remain firmly committed to our mission of building a strong and stable Afghanistan, together with our Afghan partners."

Update at 3:30 p.m. Obama phones Afghan leader

The president called Hamid Karzai to "express his shock and sadness," the White House said in a statement.

"President Obama extended his condolences to the people of Afghanistan, and made clear his Administration's commitment to establish the facts as quickly as possible and to hold fully accountable anyone responsible," the statement said. "The President reaffirmed our deep respect for the Afghan people and the bonds between our two countries."

Update at 2:11 p.m. ET: Panetta: 'We Will Spare no Effort'

Secretary of Defense Leon Panetta has reached out to President Hamid Karzai to offer his "deepest condolences and profound regret."

"I gave President Karzai my assurances that we will bring those responsible to justice," Panetta said in a statement released by his office.

"We will spare no effort in getting the facts as quickly as possible, and we will hold any perpetrator who is responsible for this violence fully accountable under the law."

Americans share the outrage of the Afghan people, Panetta said. "This tragic incident does not reflect the commitment of the U.S. military to protect the Afghan people and help build a strong and stable Afghanistan."

Shortly afterwards, President Obama echoed Panetta's statement:

"I am deeply saddened by the reported killing and wounding of Afghan civilians. I offer my condolences to the families and loved ones of those who lost their lives, and to the people of Afghanistan, who have endured too much violence and suffering. This incident is tragic and shocking, and does not represent the exceptional character of our military and the respect that the United States has for the people of Afghanistan. I fully support Secretary Panetta's and General Allen's commitment to get the facts as quickly as possible and to hold accountable anyone responsible."

Update at 12:49 p.m. ET: ISAF Spokesperson: 'This Is A Legal Case'

As things look right now, Brig. Gen. Carsten Jacobson said at a news conference, the shooting spree "looks very much like an individual act."

The AP reports that outraged villagers want the U.S. to hand over the detained soldier, but Jacobson made it clear that the investigation will be on American terms.

"This, of course, is a legal case," Jacobson said. "He is a U.S. soldier and it will fall to the United States authorities to investigate this case, but obviously ISAF is doing what is possible at the moment to deal with the consequences."

Update at 11:03 a.m. ET: Another Crisis For U.S.-Afghan Relations

Details are still trickling in about what exactly happened, but as NPR's Quil Lawrence tells Weekend Edition Sunday, the soldier detained by NATO allegedly went on a house-to-house shooting spree in the Panjwai district of Kandahar. Afterwards, the service member returned to base and surrendered. It's not clear what triggered the shootings or what the soldier's motives were.

Relations between the United States and Afghanistan had been slowly returning to normal after last month's accidental burning of the Koran at an American military base. But this morning's news may erase that progress.

"We had just been recovering from what people were calling the lowest point in U.S.-Afghan relations," Lawrence says.

"People had just been getting back to meetings and getting off lockdown after the security situation. We haven't seen the ripple effect across the country — the news is just breaking out — but people here are very worried that this could undo the progress that was made in the last couple of weeks."

Update at 10:38 a.m. ET.: Karzai Says Women And Children Among Dead

The AP is alerting Afghanistan President Hamid Karzai is saying nine children and three women are among the dead.

Karzai called the attack "an assassination" and demanded an explanation from the United States.

Update at 7:56 a.m. ET. New ISAF Statement:

In a new statement, the ISAF deputy commander, Lt. Gen. Adrian J. Bradshaw expresses his "profound regrets and dismay." Unlike an earlier statement, this one explicitly mentions reports of deaths:

"One of our soldiers is reported to have killed and injured a number of civilians in villages adjacent to his base. I cannot explain the motivation behind such callous acts, but they were in no way part of authorized ISAF military activity. An investigation is already underway and every effort will be made to establish the facts and hold anyone responsible to account."

Update at 7:40 a.m. ET. Reports On Death Toll:

While local officials have reported deaths, NATO has yet to confirm any. The number of dead varies from around 10 to more than 15. The New York Times reports at least 15 were killed, citing "local villagers and provincial officials." According to the BBC, 10 were killed and five wounded, though it says "the death toll could rise as high as 17."

"He walked right off base, started shooting civilians, and returned to the base and turned himself in," Waggnor says.

The U.S. embassy in Kabul has issued a word of caution to residents and travelers in Kandahar province via Twitter, "due to reports of an on-going protest in Panjwa'i," the district where the shooting took place.

"A U.S. soldier opened fire on civilians after wandering off his military base in the Panjwai district of Kandahar province."

Update at 3:32 a.m. ET. Continued Tension:

Kandahar Gov. Tooryalai Wesa says people were killed and wounded in the shooting in Kandahar province, according to the AP. But NATO spokesman Capt. Justin Brockhoff said the coalition did not have reports of any deaths, but "multiple wounded."

As Al-Jazeera reports, "civilian casualties have been a major source of friction" between the Afghan government and U.S.-led NATO forces in the country. The news agency adds:

"Anti-American sentiment had already been running high before news of the latest civilian casualties."

Recent anti-American protests and violence in Afghanistan follow international forces' burning of Qurans, which were reported Feb. 21. U.S. officials, including President Obama, have apologized for the incident. The U.S. military says the religious texts were mistakenly mixed with trash.

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